Hydraulic driving unit

ABSTRACT

A hydraulic driving unit is provided with a directional control valve  23  and directional control valve  24  for controlling a boom cylinder  6  and arm cylinder  7 , both of which are driven by pressure oil delivered from a main hydraulic pump, respectively, a boom control device  25  for selectively controlling the directional control valve  23  and an arm control device  26  for selectively controlling the directional control valve  24 , and is mounted on a hydraulic excavator. To permit effective use of pressure oil in a rod chamber  6   a  of the boom cylinder  6  when a bottom pressure of the arm cylinder  7  has become high while performing a combined operation such that pressure oil is being supplied to bottom chambers  6   a,   7   a  of the boom cylinder  6  and arm cylinder  7 , the hydraulic driving unit is provided with a communication controlling means for bringing the rod chamber  6   b  of the boom cylinder  6  and the bottom chamber  7   a  of the arm cylinder  7  into communication with each other when the bottom pressure of the arm cylinder  7  has increased to a high pressure equal to or higher than a predetermined pressure.

TECHNICAL FIELD

This invention relates to a hydraulic driving unit mounted on aconstruction machine such as a hydraulic excavator to permit a combinedoperation of plural hydraulic cylinders.

BACKGROUND ART

As a hydraulic driving unit mounted on a construction machine to performcombined operations of plural hydraulic cylinders, there is known, forexample, the hydraulic driving unit disclosed in JP 2000-337307A. Thishydraulic driving unit is mounted on a hydraulic excavator. FIG. 11 is ahydraulic circuit diagram showing the construction of an essential partof the hydraulic driving unit disclosed in JP 2000-337307A, and FIG. 12is a side view illustrating a hydraulic excavator on which the hydraulicdriving unit shown in FIG. 11 is arranged.

The hydraulic excavator illustrated in FIG. 12 is provided with a travelbase 1, a revolving superstructure 2 arranged on the travel base 1, aboom 3 mounted swingably in a vertical direction on the revolvingsuperstructure 2, an arm 4 mounted swingably in a vertical direction onthe boom 3, and a bucket 5 mounted turnably in a vertical direction onthe arm 4. The boom 3, arm 4 and bucket 5 make up front attachments. Thehydraulic excavator is also provided with a boom cylinder 6 whichconstitutes a first hydraulic cylinder for driving the boom 3, an armcylinder 7 which constitutes a second hydraulic cylinder for driving thearm 4, and a bucket cylinder 8 for driving the bucket 5.

FIG. 11 shows a center-bypass hydraulic driving unit for driving theboom cylinder 6 and arm cylinder 7 in the above-mentioned hydraulicdriving units suitable for arrangement on hydraulic excavators.

As shown in FIG. 11, the boom cylinder 6 is provided with a bottomchamber 6 a and a rod chamber 6 b. By supplying pressure oil to thebottom chamber 6 a, the boom cylinder 6 is caused to extend to performboom raising. By supplying pressure oil to the rod chamber 6 b, on theother hand, the boom cylinder 6 is caused to retract to perform boomlowering. The arm cylinder 7 is also provided with a bottom chamber 7 aand rod chamber 7 b. By supplying pressure oil to the bottom chamber 8a, arm crowding is performed. By supplying pressure oil to the rodchamber 7 b, on the other hand, arm dumping is performed.

The hydraulic driving unit which includes these arm cylinder 6 and armcylinder 7 is provided with an engine 20, a main hydraulic pump 21driven by the engine 20, a boom-related, directional control valve 23 asa first directional control valve for controlling a flow of pressure oilto be supplied from the main hydraulic pump 21 to the boom cylinder 6,an arm-related, directional control valve 24 as a second directionalcontrol valve for controlling a flow of pressure oil to be supplied fromthe main hydraulic pump 21 to the arm cylinder 7, a boom control device25 as a first control device for selectively controlling theboom-related, directional control valve 23, an arm control device 26 asa second control device for selectively controlling the arm-related,directional control valve 24, and a pilot pump 22 driven by the engine20.

The boom-related, directional control valve 23 is arranged on a line 28extending to a delivery line of the main hydraulic pump 21, while thearm-related, directional control valve 24 is arranged on a line 27extending to the above-mentioned delivery line.

The boom-related, directional control valve 23 and the bottom chamber 6a of the boom cylinder 6 are connected via a main line 29 a, while theboom-related, directional control valve 23 and the rod chamber 6 b ofthe boom cylinder 6 are connected via a main line 29 b. Similarly, thearm-related, directional control valve 24 and the bottom chamber 7 a ofthe arm cylinder 7 are connected via a main line 30 a, while thearm-related, directional control valve 24 and the rod chamber 7 b of thearm cylinder 7 are connected via a main line 30 b.

The boom control device 25 is connected to the pilot pump 22. A pilotpressure produced as a result of its operation is supplied via one ofpilot lines 25 a,25 b to a corresponding control compartment of theboom-related, directional control valve 23 such that the boom-related,directional control device 23 is changed over into the left position orthe right position as viewed in FIG. 11.

Similarly, the arm control device 26 is also connected to the pilot pump22. A pilot pressure produced as a result of its operation is suppliedvia one of pilot lines 26 a,26 b to a corresponding control compartmentof the arm-related, directional control valve 24 such that thearm-related, directional control device 24 is changed over into the leftposition or the right position as viewed in FIG. 11.

In the hydraulic excavator provided with the hydraulic driving unitconstructed as described above, the boom control device 25 shown in FIG.11 is operated upon performing digging or the like of earth, and a pilotpressure is hence produced, for example, in the pilot line 25 a. Whenthe boom-related, directional control valve 23 is changed over into theleft position as viewed in FIG. 11, the pressure oil delivered from themain hydraulic pump 21 is supplied to the bottom chamber 6 a of the boomcylinder 6 via the line 28, the boom-related, directional control valve23 and the main line 29 a, while the pressure oil in the rod chamber 6 bis caused to return to a reservoir 43 via the main line 29 b and theboom-related, directional control valve 23. As a result, the boomcylinder 6 extends as indicated by arrow 13 in FIG. 12 so that the boom3 is swung as indicated by arrow 12 in FIG. 12 to perform boom raising.

Concurrently with this boom raising operation, the arm control device 26is also operated and a pilot pressure is hence produced, for example, inthe pilot line 26 a. when the arm-related, directional control valve 24is changed over into the left position as viewed in FIG. 11, thepressure oil delivered from the main hydraulic pump 21 is supplied tothe bottom chamber 7 a of the arm cylinder 7 via the line 27, thearm-related, directional control valve 24 and the main line 30 a, whilethe pressure oil in the rod chamber 7 b is caused to return to thereservoir 43 via the main line 30 b and the arm-related, directionalcontrol valve 24. As a result, the arm cylinder 7 extends as indicatedby arrow 9 in FIG. 12 so that the arm 4 is swung as indicated by arrow11 in FIG. 12 to perform arm crowding.

When an unillustrated bucket control device is also operatedconcurrently with such a boom raising and arm crowding operation tochange over a bucket-related, directional control valve such that thebucket cylinder 8 illustrated in FIG. 12 is caused to extend in thedirection of arrow 10 in FIG. 12, the bucket 5 is caused to turn in thedirection of arrow 11 to perform earth digging work or the like asdesired.

FIG. 13 contains characteristic diagrams illustrating pilot pressurecharacteristics and cylinder pressure characteristics in theabove-described combined operation. In the lower diagram of FIG. 13,time lengths of digging work are plotted along abscissas, and pilotpressures produced by the control device are plotted along ordinates. Abroken line 31 in the lower diagram of FIG. 13 indicates pilot pressuresproduced by the arm control device 26 and to be supplied to the pilotline 26 a, while a solid line 32 in the lower diagram of FIG. 13designates pilot pressures produced by the boom control device 25 and tobe supplied to the pilot line 25 a, that is, pilot pressures upon boomraising. T1, T2 and T3 indicate time points at which boom raisingoperations were performed, respectively.

In the upper diagram of FIG. 13, on the other hand, time lengths ofdigging work are plotted along abscissas, and load pressures produced inthe hydraulic cylinders 6,7, in other words, cylinder pressures areplotted along ordinates. A broken line 33 in the upper diagram of FIG.13 indicates bottom pressures produced in the bottom chamber 7 a of thearm cylinder 7, that is, arm cylinder bottom pressures, while a solidline 34 designates rod pressures produced in the rod chamber 6 b of theboom cylinder 6, that is, boom cylinder rod pressures. When such acombined operation of boom raising and arm crowding is performed, forcein the direction of arrow 12 in FIG. 12 is transmitted to the boom 3 bycounterforce produced when the bucket 5 digs earth. As a consequence,the boom cylinder 6 tends to be pulled in the direction of arrow 13 inFIG. 12, and as indicated by the boom rod pressure 34 in the upperdiagram of FIG. 13, a high pressure is produced in the rod chamber 6 bof the boom cylinder 6.

In the above-described conventional art, earth digging work or the likecan be performed without a problem by combined operations of boomraising and arm crowding. Nonetheless, it is desired to achieve moreefficient work.

The present inventors' attention was attracted to the current situationthat the pressure oil in the rod chamber 6 b of the first hydrauliccylinder as the boom cylinder 6 had been drained directly to thereservoir 43 upon performing the above-described combined operation ofboom raising and arm crowding, namely when pressure oil was supplied toboth of the bottom chambers 6 a,7 a of the first hydraulic cylinder asthe boom cylinder 6 and the second hydraulic cylinder as the armcylinder 7 and as a consequence, an operation which would lead todevelopment of a higher rod pressure in the first hydraulic cylinder asthe boom cylinder 6 was performed.

The present invention has been completed in view of the above-describedsituation of the conventional art, and as an object, has the provisionof a hydraulic driving unit which makes it possible to effectively usethe pressure oil in the rod chamber of the first hydraulic cylinder whenthe bottom pressure of the second hydraulic cylinder becomes high duringa combined operation performed by supplying pressure oil to therespective bottom chambers of the first hydraulic cylinder and secondhydraulic cylinder although the pressure oil in the rod chamber of thefirst hydraulic cylinder has heretofore been drained into the reservoir.

DISCLOSURE OF THE INVENTION

To achieve the above-described object, the invention provides ahydraulic driving unit mounted on a construction machine and providedwith a main hydraulic pump, a first hydraulic cylinder and secondhydraulic cylinder driven by pressure oil delivered from the mainhydraulic pump, a first directional control valve for controlling a flowof pressure oil to be supplied from the main hydraulic pump to the firsthydraulic cylinder, a second directional control valve for controlling aflow of pressure oil to be supplied from the main hydraulic pump to thesecond hydraulic pump, a first control device for selectivelycontrolling the first directional control valve, and a second controldevice for selectively controlling the second directional control valve,characterized in that the hydraulic driving unit is provided with acommunication control means for bringing a rod chamber of the firsthydraulic cylinder and a bottom chamber of the second hydraulic cylinderinto communication with each other when a bottom pressure of the secondhydraulic cylinder has increased to a high pressure equal to or higherthan a predetermined pressure.

According to the invention as described above, upon performing acombined operation of the first hydraulic cylinder and the secondhydraulic cylinder by operating the first control device and secondcontrol device to change over the first directional control valve andthe second directional control valve, respectively, and supplying thepressure oil from the main hydraulic pump to the respective bottomchambers of the first hydraulic cylinder and second hydraulic cylindervia the first directional control valve and the second directionalcontrol valve, the communication control means is operated to supply thepressure oil from the rod chamber of the first hydraulic cylinder to thebottom chamber of the second hydraulic cylinder when the bottom pressureof the second hydraulic cylinder has become a high pressure equal to orhigher than the predetermined pressure. Described specifically, thepressure oil delivered from the main hydraulic pump and to be suppliedvia the second directional control valve and the pressure oil suppliedfrom the rod chamber of the first hydraulic cylinder are combined andsupplied to the bottom chamber of the second hydraulic cylinder, and asa consequence, an acceleration of the second hydraulic cylinder in itsextending direction can be performed. As can be understood from theforegoing, it is possible to effectively use the pressure oil in the rodchamber of the first hydraulic cylinder, which has heretofore beendrained to the reservoir, for the selective acceleration of the secondhydraulic cylinder.

In a preferred embodiment of the invention the communication controlmeans comprises a communication line capable of bringing the rod chamberof the first hydraulic cylinder and the bottom chamber of the secondhydraulic cylinder into communication with each other, a check valvearranged on the communication line to prevent a flow of pressure oilfrom the bottom chamber of the second hydraulic cylinder toward the rodchamber of the first hydraulic cylinder, and a switching valve forcommunicating the communication line to a reservoir when the bottompressure of the second hydraulic cylinder is lower than thepredetermined pressure and for maintaining the communication line in acommunicating state when the bottom pressure of the second hydraulicpressure has become equal to or higher than the predetermined pressure.

Upon performing a combined operation of the first hydraulic cylinder andthe second hydraulic cylinder by supplying the pressure oil from themain hydraulic pump to the respective bottom chambers of the firsthydraulic cylinder and second hydraulic cylinder, the switching valve ischanged over to maintain the communication line in a communicating statewhen the bottom pressure of the second hydraulic cylinder has increasedto a high pressure equal to or higher than the predetermined, and as aresult, the pressure oil in the rod chamber of the first hydrauliccylinder is supplied to the bottom chamber of the second hydrauliccylinder via the communication line and the check valve. Describedspecifically, the pressure oil to be supplied to the bottom chamber ofthe second hydraulic cylinder via the second directional control valveand the pressure oil supplied from the rod chamber of the firsthydraulic cylinder are combined and supplied, and as a consequence, anacceleration of the second hydraulic cylinder in its extending directioncan be performed.

When the bottom pressure of the second hydraulic cylinder is a lowpressure not reaching the predetermined pressure upon performing thecombined operation of the first hydraulic cylinder and the secondhydraulic cylinder as mentioned above, the switching valve is maintainedsuch that the communication line is in communication with the reservoir.As a consequence, the pressure oil in the rod chamber of the firsthydraulic cylinder is returned to the reservoir. In this case, thebottom chamber of the second hydraulic cylinder is supplied withpressure oil only via the second directional control valve so that noacceleration is performed in the extending direction of the secondhydraulic cylinder.

In a still more preferred embodiment of the invention, the hydraulicdriving unit is provided with a detection means for detecting the bottompressure of the second hydraulic cylinder and the switching valve isoperated in accordance with the bottom pressure of the second hydrauliccylinder as detected by the detection means.

When the bottom pressure of the second hydraulic cylinder is detected bythe detection means to have increased to a high pressure equal to orhigher than the predetermined pressure, the switching valve is changedover to maintain the communication line in a communicating state. As aconsequence, the pressure oil in the rod chamber of the first hydrauliccylinder is supplied to the bottom chamber of the second hydrauliccylinder via the communication line and check valve.

In another aspect of the invention, the hydraulic driving unit isprovided with a line connected at an end thereof to an upstream side ofthe switching valve and communicated at an opposite end thereof to thereservoir and an on/off valve arranged on the line to open the lineresponsive to a predetermined operation of the first control device.

When the predetermined operation of the first control device is anoperation to supply pressure oil to the rod chamber of the firsthydraulic cylinder, the communication line is brought into communicationwith the reservoir via the on/off valve owing to an operation of theon/off valve even when the bottom pressure of the second hydrauliccylinder is a high pressure equal to or higher than the predeterminedpressure and the switching valve is changed over to maintain thecommunication line in the communicating state. It is, therefore,possible to avoid such a situation that the pressure oil in the bottomchamber of the first hydraulic cylinder would be supplied to the bottomchamber of the second hydraulic cylinder via the communication line.

Further, the first control device is a pilot control device forgenerating a pilot pressure and the on/off valve is a pilot-controlledcheck valve.

The pilot-controlled check valve is operated responsive to an operationof the pilot control device, and the communication line is brought intocommunication with the reservoir via the pilot-controlled check valve.

In still another aspect of the invention the switching valve comprises avariable restrictor.

The opening of the variable restrictor included in the switching valvevaries in accordance with the level of the bottom pressure of the secondhydraulic cylinder. Described specifically, when the bottom pressure ofthe second hydraulic cylinder is a high pressure equal to or higher thanthe predetermined pressure but is not a substantially high pressure, theopening of the variable restrictor in the switching valve becomessmaller such that the flow rate of the pressure oil to be supplied fromthe rod chamber of the first hydraulic cylinder to the communicationline through the variable restrictor can be reduced. When the bottompressure of the second hydraulic cylinder has become a very highpressure, on the other hand, the opening of the variable restrictor inthe switching valve becomes greater such that the flow rate of thepressure oil to be supplied from the rod chamber of the first hydrauliccylinder to the communication line through the variable restrictor canbe increased.

In a modification, the hydraulic driving unit is provided with a firstflow rate control means for controlling a flow rate through thecommunication line in accordance with a quantity of an operation of thesecond control device.

The flow rate through the communication line can be controlled inaccordance with the quantity of an operation of the second controldevice, which controls the second hydraulic cylinder, without relyingsolely upon the quantity of a change-over of the switching valve.Namely, the speed of the second hydraulic cylinder, which is in anaccelerated state, can be controlled in accordance with the quantity ofan operation of the second control device.

The first flow rate control means comprises a variable restrictor.

When the quantity of an operation of the second control device isrelatively small, the opening of the variable restrictor becomesrelatively small and through this small opening, the pressure oil can besupplied at a relatively low flow rate from the communication line tothe bottom chamber of the second hydraulic cylinder. As a consequence,the speed of the second hydraulic cylinder, which is in an acceleratedstate, can be rendered relatively low. When the quantity of an operationof the second control device becomes relatively great and the opening ofthe variable restrictor becomes large, the pressure oil can be suppliedat a relatively high flow rate from the communication line to the bottomchamber of the second hydraulic cylinder through the large opening. As aconsequence, the speed of the second hydraulic cylinder, which is in anaccelerated state, can be rendered relatively high.

In another preferred embodiment of the invention, the hydraulic drivingunit is provided with a second flow rate control means for controlling aflow rate through the communication line in accordance with a quantityof an operation of the first control device.

The flow rate through the communication line can also be controlled viathe second flow rate control means in accordance with the quantity of anoperation of the first control device which controls the first hydrauliccylinder. Namely, the speed of the second hydraulic cylinder, which isin an accelerated state, can be controlled in accordance with thequantity of an operation of the second control device.

it is preferred that the second flow rate control means comprises avariable restrictor.

When the quantity of an operation of the first control device isrelatively small, the opening of the variable restrictor associated withthe operation of the first control device becomes relatively small andas a result of the operation of the first control device, the pressureoil can be supplied at a relatively low flow rate from the communicationline to the bottom chamber of the second hydraulic cylinder through thissmall opening. As a consequence, the speed of the second hydrauliccylinder, which is in an accelerated state, can be rendered relativelylow. When the quantity of an operation of the first control device isrelatively great, the opening of the variable restrictor associated withthis operation of the first control device becomes relatively large, andas a result of the operation of the first control device, the pressureoil can be supplied at a relatively high flow rate from thecommunication line to the bottom chamber of the second hydrauliccylinder through the large opening. As a consequence, the speed of thesecond hydraulic cylinder, which is in an accelerated state, can berendered relatively high.

The first control device preferably is a pilot control device forgenerating a pilot pressure, the switching valve is a pilot-controlledswitching valve with a variable restrictor incorporated therein and thesecond flow rate control means comprises a control line for bringing thefirst control device and a control compartment of the pilot-controlledswitching valve into communication with each other.

When the quantity of an operation of the first control device isrelatively small, the pilot pressure applied from the first controldevice to the control compartment of the pilot-controlled switchingvalve via the control line is relative low, the opening of the variablerestrictor included in the pilot-controlled switching valve hencebecomes relatively small, and as a result of the operation of the firstcontrol device, the pressure oil can be supplied at a relatively lowflow rate from the communication line to the bottom chamber of thesecond hydraulic cylinder through this small opening. As a consequence,the speed of the second hydraulic cylinder, which is in an acceleratedstate, can be rendered relatively low. When the quantity of an operationof the first control device is relatively great, the pilot pressureapplied from the first control device to the control compartment of thepilot-controlled switching valve through the control line is relativelyhigh, the opening of the variable restrictor included in thepilot-operated switching valve hence becomes relatively large, and as aresult of the operation of the first control device, the pressure oilcan be supplied at a relatively high flow rate from the communicationline to the bottom chamber of the second hydraulic cylinder through thelarge opening. As a consequence, the speed of the second hydrauliccylinder, which is in an accelerated state, can be rendered relativelyhigh.

The communication control means comprises a bottom pressure detector fordetecting the bottom pressure of the second hydraulic cylinder andoutputting an electrical signal and a controller for outputting acontrol signal to selectively control the switching valve in accordancewith the signal outputted from the bottom pressure detector.

When the bottom pressure of the second hydraulic cylinder is detected bythe bottom pressure detector to have become a high pressure equal to orhigher than the predetermined pressure, an electrical signal outputtedfrom the bottom pressure detector is inputted to the controller. As aconsequence, a control signal to change over the switching valve isoutputted from the controller so that the switching valve is changedover to maintain the communication line in the communicating state. As aresult, the pressure oil in the rod chamber of the first hydrauliccylinder is supplied to the bottom chamber of the second hydrauliccylinder via the check valve.

The hydraulic driving unit is provided with a first operated-quantitydetector for detecting a quantity of an operation of the second controldevice and outputting an electrical signal, and the controller comprisesa first function generator for outputting a value such that the valuegradually becomes greater as the bottom pressure of the second hydrauliccylinder becomes higher, a second function generator for outputting avalue such that the value gradually becomes greater but not beyond 1 asan upper limit as the quantity of the operation of the second controldevice becomes greater, and a first multiplier for performingmultiplication to output the control signal in accordance with a signaloutputted from the first function generator and a signal outputted fromthe second function generator.

When a value which gradually becomes greater as the bottom pressure ofthe second hydraulic cylinder becomes higher is outputted from the firstfunction generator and a value corresponding to the quantity of anoperation of the second control device is outputted by the firstoperated-quantity detector, the first multiplier performs computing suchthat these values outputted from the first and second functiongenerators are multiplied with each other. A control signalcorresponding to the thus-computed value is outputted from thecontroller, and the quantity of a change-over of the switching valve iscontrolled. Namely, the speed of the second hydraulic cylinder, which isin an accelerated state, can be controlled in accordance with thequantity of an operation of the second control device.

In another embodiment of the invention the hydraulic driving unit isprovided with a second operated-quantity detector for detecting aquantity of an operation of the first control device to output anelectrical signal, and the controller comprises a third functiongenerator for outputting a value such that the value gradually becomesgreater but not beyond 1 as an upper limit as the quantity of theoperation of the first control device becomes greater and a secondmultiplier for performing multiplication to output the control signal inaccordance with a signal outputted from the first multiplier and asignal outputted from the third function generator.

When a value corresponding to the quantity of an operation of the firstcontrol device is outputted by the second operated-quantity detectorfrom the third function generator, the second multiplier performscomputing such that the value outputted from the first functiongenerator and the value outputted from the third function generator aremultiplied with each other. A control signal corresponding to thethus-computed value is outputted from the controller, and the quantityof a change-over of the switching valve is controlled. Namely, the speedof the second hydraulic cylinder, which is in an accelerated state, canbe controlled in accordance with the quantity of an operation of thefirst control device.

The switching valve is a pilot-controlled switching valve, and thehydraulic driving unit is provided with an electrohydraulic converterfor outputting a control pressure corresponding to a value of thecontrol signal outputted from the controller and a control line throughwhich the electrohydraulic converter and a control compartment of thepilot-controlled switching valve are communicated with each other.

When a control signal outputted from the controller is applied to theelectrohydraulic converter, a pilot pressure of a magnitudecorresponding to the value of the control signal is applied from theelectrohydraulic converter to the control compartment of thepilot-controlled switching valve via the control line, and the quantityof a change-over of the switching valve is controlled in accordance withthe level of the pilot pressure.

The first hydraulic cylinder and the second hydraulic cylinder comprisea boom cylinder and an arm cylinder, respectively, the first directionalcontrol valve and the second directional control valve comprise acenter-bypass directional control valve for a boom and a center-bypassdirectional control valve for an arm, respectively, and the firstcontrol device and the second control device comprise a boom controldevice and an arm control device, respectively.

When the bottom pressure of the arm cylinder increases to a highpressure equal to or higher than the predetermined pressure uponperforming a combined operation of the boom cylinder and the armcylinder, specifically a combined operation of boom raising and armcrowding by operating the boom control device and the arm control deviceto change over the boom-related, directional control valve and thearm-related, directional control valve, respectively, and supplying thepressure oil from the main hydraulic pump to the respective bottomchambers of the boom cylinder and arm cylinder via the boom-related,directional control valve and arm-related, directional control valve,the communication control means operates so that the pressure oil in therod chamber of the boom cylinder is supplied to the bottom chamber ofthe arm cylinder. Described specifically, the pressure oil deliveredfrom the main hydraulic pump and supplied via the arm-related,directional control valve and the pressure oil supplied from the rodchamber of the boom cylinder are combined and supplied to the bottomchamber of the arm cylinder, and as a result, an acceleration of the armcylinder in its extending direction, that is, an acceleration of armcrowding can be performed.

The invention finds practical application when the construction machineis a hydraulic excavator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram showing a first embodiment of thehydraulic driving unit according to the present invention.

FIG. 2 contains characteristic diagrams illustrating pilot pressurecharacteristics and cylinder flow-rate characteristics in the firstembodiment shown in FIG. 1.

FIG. 3 is a hydraulic circuit diagram showing a second embodiment of thepresent invention.

FIG. 4 is a hydraulic circuit diagram showing a third embodiment of thepresent invention.

FIG. 5 is a hydraulic circuit diagram showing a fourth embodiment of thepresent invention.

FIG. 6 is a hydraulic circuit diagram showing a fifth embodiment of thepresent invention.

FIG. 7 is a hydraulic circuit diagram showing a sixth embodiment of thepresent invention.

FIG. 8 is a block diagram illustrating the constitution of an essentialpart of a controller arranged in the sixth embodiment shown in FIG. 7.

FIG. 9 is a hydraulic circuit diagram showing a seventh embodiment ofthe present invention.

FIG. 10 is a block diagram illustrating the constitution of an essentialpart of a controller arranged in the seventh embodiment shown in FIG. 9.

FIG. 11 is a hydraulic circuit diagram showing a conventional hydraulicdriving unit.

FIG. 12 is a side view depicting a hydraulic excavator described as anexample of a construction machine on which the hydraulic driving unitshown in FIG. 11 is arranged.

FIG. 13 contains characteristic diagrams illustrating pilot pressurecharacteristics and cylinder pressure characteristics in theconventional hydraulic driving unit.

BEST MODES FOR CARRYING OUT THE INVENTION

The embodiments of the hydraulic driving unit according to the presentinvention will hereinafter be described based on the drawings.

FIG. 1 is a circuit diagram showing the first embodiment of thehydraulic driving unit according to the present invention.

In FIG. 1 and also in FIGS. 3 to 7 and 9 to be described subsequentlyherein, elements equivalent to those shown in FIG. 11 described aboveare indicated by like reference numerals. Further, the first embodimentshown in FIG. 1 and the second to seventh embodiments to be describedsubsequently herein are also arranged on construction machines, forexample, on the above-described hydraulic excavator illustrated in FIG.12. The reference numerals shown in FIG. 12 will, therefore, be referredto in the subsequent description as needed.

The first embodiment shown in FIG. 1 is designed to drive, for example,a boom cylinder 6 as a first hydraulic cylinder and an arm cylinder 7 asa second hydraulic cylinder by a similar center-bypass hydraulic drivingunit as in the above-described conventional art. Although overlappingwill occur with the description based on FIG. 11, the first embodimentshown in FIG. 1 is also constructed such that the boom cylinder 6 isprovided with a bottom chamber 6 a and a rod chamber 6 b and the armcylinder 7 is likewise provided with a bottom chamber 7 a and a rodchamber 7 b.

The first embodiment is also provided with an engine 20, a mainhydraulic pump 21 and pilot pump 22 driven by the engine 20, a firstdirectional control valve for controlling a flow of pressure oil to besupplied to the boom cylinder 6, i.e., a boom-related, directionalcontrol valve 23 of the center bypass type, a second directional controlvalve for controlling a flow of pressure oil to be supplied to the armcylinder 7, i.e., an arm-related, directional control valve 24 of thecenter bypass type. Also provided are a first control device forselectively controlling the boom-related, directional control valve 23,i.e., a boom control device 25 and a second control device forselectively controlling the arm-related, directional control valve 24,i.e., an arm control device 26.

Lines 27,28 are connected to a delivery line of the main hydraulic pump21, the arm-related, directional control valve 24 is arranged on theline 27, and the boom-related, directional control valve 23 is arrangedon the line 28.

The boom-related, directional control valve 23 and the bottom chamber 6a of the boom cylinder 6 are connected via a main line 29 a, while theboom-related, directional control valve 23 and the rod chamber 6 b ofthe boom cylinder 6 are connected via a main line 29 b. The arm-related,directional control valve 24 and the bottom chamber 7 a of the armcylinder 7 are connected via a main line 30 a, while the arm-related,directional control valve 24 and the rod chamber 7 b of the arm cylinder7 are connected via a main line 30 b.

The boom control device 25 and arm control device 26 are composed, forexample, of pilot control devices which produce pilot pressures, and areconnected to a pilot pump 22. Further, the boom control device 25 isconnected to control compartments of the boom-related, directionalcontrol valve 23 via pilot lines 25 a,25 b, respectively, while the armcontrol device 26 is connected to control compartments of thearm-related, directional control valve 24 via pilot lines 26 a,26 b,respectively.

The above-described constitution is similar to the above-describedconstitution illustrated in FIG. 11.

This first embodiment is provided especially with a communicationcontrol means for bringing a rod chamber 6 b of the boom cylinder 6,which makes up the first hydraulic cylinder, and a bottom chamber 7 a ofthe arm cylinder 7, which makes up the second hydraulic cylinder, intocommunication with each other when a bottom pressure of the arm cylinder7 has increased to a high pressure equal to or higher than apredetermined pressure. As illustrated by way of example in FIG. 1, thiscommunication control means includes a communication line 40 capable ofcommunicating the rod chamber 6 b of the boom cylinder 6 and the bottomchamber 7 a of the arm cylinder 7, a check valve 41 arranged on thecommunication line 40 to prevent a flow of pressure oil from the bottomchamber 7 a of the arm cylinder 7 toward the rod chamber 6 b of the boomcylinder 6, and a switching valve 44 for bringing the communication line40 into communication with a reservoir when the bottom pressure of thearm cylinder 7 is lower than the predetermined pressure and for bringingthe communication line 40 into a communicating state when the bottompressure of the arm cylinder 7 has become a high pressure equal to orhigher than the predetermined pressure. This switching valve 44 iscomposed of a pilot-controlled switching valve which is changed over bya control pressure. Described specifically, the communication line 40located between the check valve 41 and the bottom chamber 7 a of the armcylinder 7 is provided with a detection means for detecting the bottompressure of the arm cylinder 7, for example, a control line 45, andresponsive to a control pressure corresponding to the bottom pressure ofthe arm cylinder 7 as detected by the control line 45, the switchingvalve 44 is operated, in other words, selectively controlled.

Also arranged are a line 46 connected at an end thereof to thecommunication line 40 located on an upstream side of the check valve 41and at an opposite end thereof to a reservoir 43; and an on/off valve,for example, a pilot-controlled check valve 47 arranged on the line 46such that responsive to a predetermined operation of the boom controldevice as the first control device, for example, an operation to supplypressure oil to the pilot line 25 b to perform boom lowering, the line46 is opened. The above-described pilot line 25 b and pilot-controlledcheck valve 47 are connected together by a control line 48.

In the first embodiment constituted as described above, combinedoperations of the boom cylinder 6 and the arm cylinder 7 are performedas will be described hereinafter.

[Combined Operation of Boom Raising and Arm Crowding]

When the boom control device 25 is operated to supply a pilot pressureto the pilot line 25 a such that the boom-related, directional controlvalve 23 is changed over into the left position shown in FIG. 1 andfurther, the arm control device 26 is operated to supply a pilotpressure to the pilot line 26 a such that the arm-related, directionalcontrol valve 24 is changed over into the left position shown in FIG. 1,pressure oil delivered from the main hydraulic pump 21 is supplied tothe bottom chamber 6 a of the boom cylinder 6 via the line 28, theboom-related, directional control valve 23 and the main line 29 a, andfurther, the pressure oil delivered from the main hydraulic pump 21 isalso supplied to the bottom chamber 7 a of the arm cylinder 7 via theline 27, the arm-related, directional control valve 24 and the main line30 a. As a result, the boom cylinder 6 and arm cylinder 7 are bothoperated in extending directions so that as shown in FIG. 12, the boom 3and arm 4 are caused to swing in the directions of arrows 12 and 11,respectively, to perform a combined operation of boom raising and armcrowding.

During the above-described combined operation, the pilot line 25 b ofthe boom operating system is not supplied with the pilot pressure, andremains under the same pressure as the reservoir pressure. Therefore,the control line 48 takes the reservoir pressure so that thepilot-controlled check valve 47 remains in a closed position to preventcommunication between the communication line 40 and the reservoir 43 viathe line 46.

In a state that the bottom pressure of the arm cylinder 7 is lower thanthe predetermined pressure, on the other hand, the force of a controlpressure applied to the control compartment of the switching valve 44via the communication line 40 and the control line 45 is smaller thanthe spring force, and therefore, the switching valve 44 is held in theright position shown in FIG. 1. In this state, the rod chamber 6 b ofthe boom cylinder 6 is maintained in communication with the reservoir 43via the main line 29 b, the boom-related, directional control valve 23,a reservoir line 42, and the switching valve 44. During an extendingoperation of the boom cylinder 6, the pressure oil in the rod chamber 6b of the boom cylinder 6 is, therefore, returned to the reservoir 43,and the pressure oil in the rod chamber 6 b is not supplied to thecommunication line 40.

When the bottom pressure of the arm cylinder 7 rises to a high pressureequal to or higher than the predetermined pressure from such a state asdescribed above, the force of a control pressure applied to the controlcompartment of the switching valve 44 via the communication line 40 andthe control line 45 becomes greater than the spring force so that theswitching valve 44 is changed over into the left position in FIG. 1.When this state is established, the reservoir line 42 is cut off by theswitching valve 44 so that the pressure oil, which has been guided fromthe rod chamber 6 b of the boom cylinder 6 into the main line 29 b, theboom-related, directional control valve 23 and the reservoir line 42, issupplied to the communication line 40 via the check valve 41. Thepressure oil supplied to the communication line 40 is supplied to thebottom chamber 7 a of the arm cylinder 7 via the main line 30 a. Namely,the pressure oil delivered from the main hydraulic pump 21 via thearm-related, directional control valve 24 and the pressure oil suppliedfrom the rod chamber 6 b of the boom cylinder 6 are combined andsupplied to the bottom chamber 7 a of the arm cylinder 7. As a result,an acceleration of arm cylinder 6 in the extending direction can beachieved. In other words, the operating speed of arm crowding can berendered faster.

FIG. 2 contains characteristic diagrams showing pilot pressurecharacteristics and cylinder flow-rate characteristics in the firstembodiment illustrated in FIG. 1.

In FIG. 2, the lower diagram is similar to that shown in FIG. 13described above. In the upper diagram, a solid line 49 indicates adelivery flow rate from the rod chamber 6 a of the boom cylinder 6, analternate long and short dash line 50 designates the rate of an inflowinto the bottom chamber 7 a of the arm cylinder 7 as obtained by thefirst embodiment, and a broken line 51 represents the rate of an inflowinto the bottom chamber 7 a of the arm cylinder 7 in the above-describedconventional art illustrated in FIGS. 11 to 13. As evident from FIG. 2,compared with the conventional art, the first embodiment can increasethe rate of an inflow into the bottom chamber 7 a of the arm cylinder 7,and as mentioned above, can perform an acceleration in arm crowding.

[Boom Lowering and Arm Crowding Operation]

When the boom control device 25 is operated to supply a pilot pressureto the pilot line 25 b such that the boom-related, directional controlvalve 23 is changed over into the right position shown in FIG. 1 andfurther, the arm control device 26 is operated to supply a pilotpressure to the pilot line 26 a such that the arm-related, directionalcontrol valve 24 is changed over into the left position, pressure oildelivered from the main hydraulic pump 21 is supplied to the rod chamber6 b of the boom cylinder 6 via the line 28, the boom-related,directional control valve 23 and the main line 29 b, and as mentionedabove, the pressure oil delivered from the main hydraulic pump 21 isalso supplied to the bottom chamber 7 a of the arm cylinder 7 via theline 27, the arm-related, directional control valve 24 and the main line30 a. As a result, the boom cylinder 6 is operated in a retractingdirection and the arm cylinder 7 is operated in the extending direction,so that the boom 3 is caused to swing in a lowering direction oppositeto arrow 12 in FIG. 12 and the arm 4 is caused to swing in the directionof arrow 11. A combined operation of boom lowering and arm crowding isperformed, accordingly.

As the pilot pressure is being supplied to the pilot line 25 b in theboom operating system during such a combined operation, a controlpressure is guided into the control line 48 so that the pilot-controlledcheck valve 47 is brought into an open position to bring the line 46 andthe reservoir line 42 into a communicated state.

Even when the bottom pressure of the arm cylinder 7 rises to a highpressure equal to or higher than the predetermined pressure and theswitching valve 44 is changed over into the left position in FIG. 1 tobring the bottom chamber 6 a of the boom cylinder 6 and thecommunication line 40 into a communicated state via the boom-related,directional control valve 23, the bottom chamber 6 a of the boomcylinder 6 is brought into a state communicated with the reservoir 43because the reservoir line 42 and the line 46 are in the communicatedstate as mentioned above.

In this state, the pressure oil in the bottom chamber 6 a of the boomcylinder 6 is returned to the reservoir 43 via the main line 29 a andthe boom-related, directional control valve 23. The pressure oil in thebottom chamber 6 a of the boom cylinder 6 is, therefore, not supplied tothe bottom chamber 7 a of the arm cylinder 7 via the communication line40 so that no acceleration is performed in arm crowding.

Upon performing a combined operation including arm dumping in whichpressure oil is supplied to the rod chamber 7 b of the arm cylinder 7,the bottom chamber 7 a of the arm cylinder 7 is brought intocommunication with the reservoir 43. No pressure is, therefore,developed in the communication line 40 so that no acceleration of thearm cylinder 7 is performed.

In the first embodiment constructed as described above, the pressure oilin the rod chamber 6 b of the boom cylinder 6 can be combined to that inthe bottom chamber 7 a of the arm cylinder 7 during a combined operationof boom raising and arm crowding, said combined operation beingfrequently performed during digging work or the like of earth. Thismakes it possible to effectively use the pressure oil in the rod chamber6 b of the boom cylinder 6, said pressure oil having heretofore beensimply drained into the reservoir 43, for the acceleration of the armcylinder 7 and hence, to achieve an improvement in the efficiency of thework.

Even when the bottom pressure of the arm cylinder 7 is a high pressureequal to or higher than the predetermined pressure, an acceleration ofthe arm cylinder 7, in other words, an acceleration of the operatingspeed of arm crowding can be reduced by opening the pilot-controlledcheck valve 47 when boom lowering which requires retraction of the boomcylinder 6 is performed. It is, therefore, possible to continue thedesired working performance by the combined operation of boom loweringand arm crowding.

FIG. 3 is a hydraulic circuit diagram showing the second embodiment ofthe present invention.

The second embodiment is constituted especially such that a switchingvalve 52, which maintains the communication line 40 in a communicatingstate when the bottom pressure of the arm cylinder 7 as the secondhydraulic cylinder has increased to a high pressure equal to or higherthan the predetermined pressure, includes a variable restrictor 53. Theremaining constitution is similar to the corresponding constitution inthe above-described first embodiment shown in FIG. 1.

According to the second embodiment constituted as described above,similar advantageous effects are obtained as in the above-describedfirst embodiment, and in addition, the opening of the variablerestrictor 53 incorporated in the switching valve 52 varies inaccordance with the level of the bottom pressure of the arm cylinder 7.Described specifically, when the bottom pressure of the arm cylinder 7is relatively low although it is equal to or higher than thepredetermined pressure, the opening of the variable restrictor 53 in theswitching valve 52 becomes greater so that a major portion of thepressure oil from the rod chamber 6 b of the boom cylinder 6 is returnedto the reservoir 43 through the variable restrictor 53. In other words,the flow rate of the pressure oil from the rod chamber 6 b of the boomcylinder 6, said pressure oil being to be supplied to the communicationline 40, is low so that the speed of the arm cylinder 7 is limited onlyto a slight increase. When the bottom pressure of the arm cylinder 7 isa high pressure equal to or higher than the predetermined pressure andis relatively high, on the other hand, the opening of the variablerestrictor 53 in the switching valve 52 becomes smaller so that the flowrate of the pressure oil to be supplied from the rod chamber 6 b of theboom cylinder 6 to the communication line 40 becomes higher to make thespeed of the arm cylinder 7 still higher.

It is, therefore, possible to supply a flow rate, which depends upon thelevel of the bottom pressure of the arm cylinder 7, for the accelerationof the arm cylinder 7 via the communication line 40 and also to preventdevelopment of a shock which would otherwise take place as a result of asudden change in the speed of the arm cylinder 6 during acceleration.

FIG. 4 is a hydraulic circuit diagram showing the third embodiment ofthe present invention.

This third embodiment is provided especially with a first flow-ratecontrolling means for controlling a flow rate through the communicationline 40 in accordance with the quantity of an operation of the armcontrol device as the second control device. This first flow-ratecontrolling means is constituted including a variable restrictor 54 anda control line 55. The variable restrictor is interposed, for example,at an intermediate point of the communication line 40 through which thecheck valve 41 and the bottom chamber 7 a of the arm cylinder 7 arecommunicated with each other, and a control line communicates thevariable restrictor 54 and the pilot line 26 a in the arm operatingsystem with each other. The remaining constitution is similar to thecorresponding constitution in the above-described first embodimentdepicted in FIG. 1.

According to the third embodiment constituted as described above,advantageous effects equivalent to those of the above-described firstembodiment can be obtained. In addition and in particular, the flow ratethrough the communication line 40 can be controlled in accordance withthe quantity of an operation of the arm control device 26, whichoperates the arm cylinder 6, via the variable restrictor 54 withoutrelying solely upon the quantity of a change-over of the switching valve44. When the quantity of an operation of the arm control device 26 isrelatively small upon performing arm crowding, for example, the controlpressure applied to the variable restrictor 54 via the pilot line 26 aand the control line 55 is small, and accordingly, the opening of thevariable restrictor 54 becomes relatively small. Through this smallopening, the pressure oil is supplied at a relatively small flow ratefrom the communication line 40 to the bottom chamber 6 a of the armcylinder 6. As a result, the speed of the arm cylinder 6, which is in anaccelerated state, can be made relatively low. When the quantity of anoperation of the arm control device 26 becomes relatively large duringan arm crowding operation, the control pressure applied to the variablerestrictor 54 becomes higher, and the opening of the variable restrictor54 becomes greater correspondingly. Through this large opening, thepressure oil is supplied at a high flow rate from the communication line40 to the bottom chamber 6 a of the arm cylinder 6. As a result, thespeed of the arm cylinder 6, which is in an accelerated state, can bemade faster.

Namely, an acceleration of the arm cylinder 7 can be achieved inaccordance with the quantity of an operation of the arm control device26. An arm crowding operation can be performed by smoothly acceleratingthe arm cylinder 7 such that the arm crowding operation becomesconsistent with the operator's feeling of operation.

FIG. 5 is a circuit diagram showing the fourth embodiment of the presentinvention.

This fourth embodiment is constituted especially such that the hydraulicdriving unit is provided with a second flow-rate controlling means tocontrol a flow rate through the communication line 40 in accordance withthe quantity of an operation of the boom control device 25 as the firstcontrol device. This second flow-rate controlling means is constitutedincluding, for example, a branch line 57, a variable restrictor 59 and acontrol line 60. The branch line is connected at an end thereof to themain line 29 b, which communicates the boom-related, directional controlvalve 23 and the rod chamber 6 b of the boom cylinder 6 with each other,and at an opposite end thereof to a switching valve 57, the variablerestrictor is arranged on the branch line 56, and the control line isconnected at an end thereof to the pilot line 25 a in the boom operationsystem and at an opposite end thereof to the variable restrictor 59.

Further, the switching valve 57 is interposed in the reservoir line 42,and is also interposed at a point of connection between the branch line56 and the communication line 40.

The fourth embodiment is also provided with a bypass line 61, an on/offvalve, for example, a pilot-controlled check valve 62 arranged on thebypass line 61, and a control line 63 connected at an end thereof to thepilot line 25 b in the boom operating system and at an opposite endthereof to the pilot-controlled check valve 62. The bypass linecommunicates a reservoir line 42, said drain-line being located on anupstream side of the switching valve 57, and the reservoir line 42, saiddrain-line being located on a downstream side of the switching valve 57,with each other. In FIG. 5, numeral 58 indicates a control line whichconstitutes a detection means for detecting the bottom pressure of thearm cylinder 7.

The remaining constitution is similar to the corresponding constitutionin the above-described third embodiment depicted in FIG. 4.

In the fourth embodiment constituted as described above, similaradvantageous effects are obtained as in the above-described thirdembodiment shown in FIG. 4. In addition and in particular, the flow ratethrough the communication line 40 can also be controlled in accordancewith the quantity of an operation of the boom control device 25 whichcontrols the boom cylinder 6. When during a combined operation of boomraising and arm crowding, for example, the bottom pressure of the armcylinder 7 rises to a high pressure equal to or higher than thepredetermined pressure, the switching valve 57 is in a state changedover in the right position of FIG. 5 and communicating the branch line56 and the communication line 40 via the switching valve 57, and thequantity of an operation of the boom control device 25 is relativelysmall, a control pressure to be applied to the variable restrictor 59via the pilot line 25 a and the control line 60 as a result of theoperation of the boom control device 25 is relatively low. As aconsequence, the opening of the variable restrictor 59 becomesrelatively small so that through this small opening, the pressure oil inthe rod chamber 6 b of the boom cylinder 6 can be supplied at arelatively low flow rate to the bottom chamber 7 a of the arm cylinder 7via the branch line 56, the variable restrictor 59, the switching valve57, the check valve 41 and the communication line 40. As a result, thespeed of the arm cylinder 7, which is in an accelerated state, can bemade relatively low.

When during the above-mentioned combined operation of boom raising andarm crowding, the bottom pressure of the arm cylinder 7 rises to a highpressure equal to or higher than the predetermined pressure, theswitching valve 57 is in a state changed over in the right position ofFIG. 5, and the quantity of an operation of the boom control device 25is relatively large, a control pressure to be applied to the variablerestrictor 59 as a result of the operation of the boom control device 25becomes high. As a consequence, the opening of the variable restrictor59 becomes large so that through this large opening, the pressure oil inthe rod chamber 6 b of the boom cylinder 6 can be supplied at a highflow rate to the bottom chamber 7 a of the arm cylinder 7 via the branchline 56, the variable restrictor 59, the switching valve 57, the checkvalve 41 and the communication line 40. As a result, the speed of thearm cylinder 7, which is in an accelerated state, can be made faster.

Namely, in this fourth embodiment, an acceleration of the arm cylinder 7can also be achieved not only in accordance with the quantity of anoperation of the arm control device 26 but also in accordance with thequantity of an operation of the boom control device 25. A combinedoperation of arm raising and arm crowding can be performed by smoothlyaccelerating the arm cylinder 7 such that the combined operation of armraising and arm crowding becomes more consistent with the operator'sfeeling of operation.

When during a combined operation of boom lowering and arm crowding, thebottom pressure of the arm cylinder 7 rises to a high pressure equal toor higher than the predetermined pressure, the switching valve 57 is ina state ready for being changed over into the right position in FIG. 5,and the boom control device 25 is operated to apply a control pressureto the pilot-controlled variable restrictor 62 via the pilot line 25 band the control line 63, the pilot-controlled variable restrictor 62 isopened such that the pressure oil in the bottom chamber 6 a of the boomcylinder 6 is returned to the reservoir 43 via the main line 29 a, theboom-related, directional control valve 23, the reservoir line 42, theline 61 and the pilot-controlled check valve 62. It is, therefore,possible to perform a retracting operation of the boom cylinder 6, thatis, a boom lowering operation as desired.

As the pilot line 25 a in the boom operating system is brought into thesame pressure as the reservoir pressure in this case, the control line60 is also brought to the reservoir pressure and hence, the variablerestrictor 59 is closed. Therefore, the pressure oil in the rod chamber6 b of the boom cylinder 6 is not combined to the pressure oil in thebottom chamber 7 a of the arm cylinder 7.

FIG. 6 is a hydraulic circuit diagram showing the fifth embodiment ofthe subject invention.

This fifth embodiment is constituted especially such that the secondflow-rate controlling means, which controls the flow rate through thecommunication line 40 in accordance with the quantity of an operation ofthe boom control device 25 as the first control device, includes, forexample, a variable restrictor 64 a arranged in a switching valve 64 andalso a control line 65 through which the pilot line 25 a in the boomoperating system and the control compartment of the switching valve 64are communicated with each other. The remaining constitution isequivalent to the corresponding constitution in the above-describedfourth embodiment depicted in FIG. 5.

Similarly to the fourth embodiment illustrated in FIG. 5, the fifthembodiment constituted as described above can also control the flow ratethrough the communication line 40 in accordance with the quantity of anoperation of the boom control device 25 which operates the boom cylinder6.

When during a combined operation of boom raising and arm crowding, inparticular, the bottom pressure of the arm cylinder 7 rises to a highpressure equal to or higher than the predetermined pressure, theswitching valve 64 is in a state immediately before its being changedover into the right position in FIG. 6, and the quantity of an operationof the boom control device 25 is relatively small, a control pressure tobe applied to the corresponding control compartment of the switchingvalve 64 via the pilot line 25 a and the control line 65 as a result ofthe operation of the boom control device 25 is relatively low. As aconsequence, the quantity of a change-over of the switching valve 64 issmall, and the opening of the variable restrictor 64 a included in theswitching valve 64 becomes relatively small. Through this small opening,the pressure oil in the rod chamber 6 b of the boom cylinder 6 can besupplied at a relatively low flow rate to the bottom chamber 7 a of thearm cylinder 7 via the branch line 56, the variable restrictor 64 a inthe switching valve 64, the check valve 41 and the communication line40. As a result, the speed of the arm cylinder 7, which is in anaccelerated state, can be made relatively low.

When the quantity of an operation of the boom control device 25 isrelatively large, a control pressure to be applied to the controlcompartment of the switching valve 64 as a result of the operation ofthe boom control device 25 becomes high. As a consequence, the openingof the variable restrictor 64 a in the switching valve 64 becomes large.Through this large opening, a majority of the pressure oil in the rodchamber 6 b of the boom cylinder 6 can be supplied to the bottom chamber7 a of the arm cylinder 7. As a result, the speed of the arm cylinder 7,which is in an accelerated state, can be made faster.

In the fifth embodiment constituted as described above, similaradvantageous effects are also obtained as in the above-described fourthembodiment.

In this fifth embodiment, even when during a combined operation of boomlowering and arm crowding, the bottom pressure of the arm cylinder 7rises to a high pressure equal to or higher than the predeterminedpressure, the switching valve 64 is in a state immediately before itsbeing changed over into the right position in FIG. 6, the pilot line 25a in the boom operating system is brought into the same pressure as thereservoir pressure and the variable restrictor 64 a in the switchingvalve 64 is closed. Therefore, the pressure oil in the rod chamber 6 bof the boom cylinder 6 is not combined to the pressure oil in the bottomchamber 7 a of the arm cylinder 7.

FIG. 7 is a hydraulic circuit diagram showing the sixth embodiment ofthe subject invention, and FIG. 8 is a block diagram illustrating theconstitution of an essential part of a controller arranged in the sixthembodiment shown in FIG. 7.

The sixth embodiment shown in these FIGS. 7 and 8 is provided with acommunication controlling means for communicating the rod chamber 6 b ofthe boom cylinder 6 as the first hydraulic cylinder with the bottomchamber 7 a of the arm cylinder 7 as the second hydraulic cylinder whenthe bottom pressure of the arm cylinder 7 has risen to a high pressureequal to or higher than the predetermined pressure. The communicationcontrolling means is constituted with a bottom pressure detector 66arranged on the communication line 40 to detect a bottom pressure of thearm cylinder 7 and to output an electrical signal, a controller 68 foroutputting a control signal to selectively control the switching valve44 in response to the signal outputted from the bottom pressure detector66, an electrohydraulic converter 69 for outputting a control pressurecorresponding to the value of the control signal outputted from thecontroller 68, and a control line 57 a communicating theelectrohydraulic converter 69 and the control compartment of theswitching valve 44 with each other.

On the pilot line 26 a in the arm control system, a firstoperation-quantity detector for detecting the quantity of an operationof the arm control device 26 as the second control device and outputtingan electrical signal, that is, an arm pilot pressure detector 67 is alsoarranged.

As illustrated in FIG. 8, the controller 68 includes a first functiongenerator 68 a, a second function generator 68 b and a first multiplier8 c. The first function generator outputs a value, which becomesgradually greater as the bottom pressure of the arm cylinder 7 rises.The second function generator outputs a value, which becomes graduallygreater but not beyond 1 as an upper limit as the quantity of anoperation of the arm control device 26 increases. The first multipliermultiplies a signal, which is outputted from the first functiongenerator 68 a, with a signal outputted from the second functiongenerator 68 b.

The remaining constitution is equivalent to the correspondingconstitution in the above-described first embodiment depicted in FIG. 1.

In the sixth embodiment constituted as described above, when uponperforming a combined operation of boom raising and arm crowding inparticular, the boom control device 25 is operated to supply a pilotpressure to the pilot line 25 a such that the boom-related, directionalcontrol valve 23 is changed over into the left position as shown in FIG.7 and the arm control device 26 is operated to supply a pilot pressureto the pilot line 26 a such that the arm-related, directional controlvalve 24 is changed over into the left position, pressure oil deliveredfrom the main hydraulic pump 21 is supplied to the bottom chamber 6 a ofthe boom cylinder 6 and also to the bottom chamber 7 a of the armcylinder 7. As a result, the boom cylinder 6 and arm cylinder 7 are bothoperated in the extending directions so that the combined operation ofboom raising and arm crowding is performed.

During this combined operation, the pilot line 25 b of the boomoperating system is not supplied with the pilot pressure, and remainsunder the same pressure as the reservoir pressure. Therefore, thecontrol line 48 takes the reservoir pressure so that thepilot-controlled check valve 47 remains in a closed position to preventcommunication between the communication line 40 and the reservoir 43 viathe line 46.

When the bottom pressure of the arm cylinder 7 is of a level lower thanthe predetermined pressure, a signal value detected at the arm bottompressure detector 66 is small so that a signal value outputted from thefirst function generator 68 a to the first multiplier 68 c in thecontroller 68 shown in FIG. 8 is small. If the quantity of the operationof the arm control device 26 is small at this time, the signal valuedetected at the arm pilot pressure detector 67 is small. At the firstmultiplier 68 c, the relatively small signal values are multiplied witheach other, and a control signal of the small value is outputted fromthe controller 68 to the electrohydraulic converter 69. Theelectrohydraulic converter 69 outputs a relatively low control pressureto the control line 57 a. In this state, the force applied by thecontrol pressure to the control compartment of the switching valve 44 issmaller than the spring force so that the switching valve 44 is held inthe right position shown in FIG. 7. The pressure oil in the rod chamber6 b of the boom cylinder 6 is, therefore, not supplied to thecommunication line 40 during the extending operation of the boomcylinder 6.

When the bottom pressure of the arm cylinder 7 rises to a high pressureequal to or higher than the predetermined pressure from such a state asdescribed above, the signal value detected at the arm bottom pressuredetector 66 becomes greater so that the signal value outputted from thefirst function generator 68 a to the first multiplier 68 c in thecontroller 68 shown in FIG. 8 becomes greater. If the quantity of theoperation of the arm control device 26 becomes greater at this time, thesignal value detected at the arm pilot pressure detector 67 becomesgreater, and the signal value outputted from the second functiongenerator 68 b to the first multiplier 68 c becomes greater. At thefirst multiplier 68 c, the large signal values are, therefore,multiplied with each other, and a control signal of a large value isoutputted from the controller 68 to the electrohydraulic converter 69.Responsive to this, the electrohydraulic converter 69 outputs a highcontrol pressure to the control line 57 a. As a result, the force whichis applied by the control pressure to the control compartment of theswitching valve 44 becomes greater than the spring force so that theswitching valve 44 is changed over into the left position shown in FIG.7. When this state is reached, the reservoir line 42 is cut off by theswitching valve 44, and the pressure oil which has been guided to themain line 29 a, the boom-related, directional control valve 23 and thereservoir line 42 from the rod chamber 6 b of the boom cylinder 6 issupplied to the communication line 40 via through the check valve 41.This pressure oil supplied from the communication line 40 is supplied tothe bottom chamber 7 a of the arm cylinder 7 via the main line 30 a.Namely, the pressure oil supplied via the arm-related, directionalcontrol valve 24 and the pressure oil supplied from the rod chamber 6 bof the boom cylinder 6 are combined and supplied to the bottom chamber 7a of the arm cylinder 7. As a result, an acceleration of the armcylinder 6 in its extending direction can be performed, and theoperating speed of arm crowding can be made faster.

In the sixth embodiment constituted as described above, the pressure oilin the rod chamber 6 b of the boom cylinder 6 can also be effectivelyused for the acceleration of the arm cylinder 7 as in theabove-described first embodiment shown in FIG. 1 although the pressureoil has heretofore been returned to the reservoir 43. It is, therefore,possible to achieve an improvement in the efficiency of work.

In this sixth embodiment, the acceleration of the arm cylinder 7 can beachieved corresponding to the quantity of an operation of the armcontrol device 26 on the basis of the functional relation of the secondfunction generator 68 b in the controller 68. An arm crowding operationcan, therefore, be performed by smoothly accelerating the arm cylinder 7such that the arm crowding operation becomes consistent with theoperator's feeling of operation.

FIG. 9 is a hydraulic circuit diagram showing the seventh embodiment ofthe subject invention, and FIG. 10 is a block diagram illustrating theconstitution of an essential part of a controller arranged in theseventh embodiment.

The seventh embodiment shown in these FIGS. 9 and 10 is provided with abottom pressure detector 66, an electrohydraulic converter 69 and an armpilot pressure detector 67 making up the first operated-quantitydetector, all of which are similar to the corresponding elementsdescribed above in connection with the sixth embodiment. In addition,the pilot line 25 a in the boom operating system is provided with asecond operated-quantity detector for detecting the quantity of anoperation of the boom control device 25 as the first control device andoutputting an electrical signal, that is, a boom pilot pressure detector70.

The controller 68, on the other hand, includes not only the firstfunction generator 68 a, the second function generator 68 b and thefirst multiplier 68 c as in the above-described sixth embodiment butalso a third function generator 68 d and a second multiplier 68 e. Thisthird function generator outputs a value, which increases gradually butnot beyond 1 as an upper limit as the quantity of an operation of theboom control device 25 as the first control device becomes greater. Thesecond multiplier multiplies a signal, which is outputted from the firstmultiplier 68 c, with a signal outputted form the third functiongenerator 68 d.

The remaining constitution is similar to the corresponding constitutionin the above-described fourth embodiment depicted in FIG. 5.

The seventh embodiment constituted as described above can also bringabout equivalent advantageous effects to the above-described fourthembodiment depicted in FIG. 5 or the above-described sixth embodimentillustrated in FIG. 7. In addition and in particular, an acceleration ofthe arm cylinder 7 can also be achieved corresponding to the quantity ofan operation of the boom control device 25 on the basis of thefunctional relation of the third function generator 68 d in thecontroller 68. A combined operation of arm raising and arm crowding can,therefore, be performed by smoothly accelerating the arm cylinder 7 suchthat the combined operation becomes more consistent with the operator'sfeeling of operation.

In each of the above-described embodiments, the first hydraulic cylindercomprises the boom cylinder 6 and the second hydraulic cylindercomprises the arm cylinder 7. The second hydraulic cylinder can,however, comprise the bucket cylinder 8 illustrated in FIG. 12. In thiscase, an acceleration of the bucket cylinder 8 can be achieved.

In the above description, the present invention was applied to thecenter-bypass hydraulic driving units. However, the present invention isnot limited to such applications, and can be constituted such that it isapplicable to hydraulic driving units equipped with closed-centerdirectional control valves.

INDUSTRIAL APPLICABILITY

During a combined operation performed by supplying pressure oil torespective bottom chambers of a first hydraulic cylinder and secondhydraulic cylinder, the pressure oil in the rod chamber of the firsthydraulic cylinder has heretofore been returned to a reservoir when thebottom pressure of the second hydraulic cylinder becomes high. Accordingto the invention as described in each claim of the subject application,the pressure oil in the rod chamber of the first hydraulic cylinder canbe effectively used for the acceleration of the second hydrauliccylinder in its extending direction, and therefore, an improvement canbe achieved in the efficiency of work performed by such a combinedoperation of these first hydraulic cylinder and second hydrauliccylinder.

According to the invention as described in claim 4 or 5, even when thebottom pressure of the second hydraulic cylinder is a high pressureequal to or higher than a predetermined pressure, the second hydrauliccylinder can be prevented from an acceleration in the case of anoperation to cause retraction of the first hydraulic cylinder. A desiredwork performance, which does not require an acceleration of the secondhydraulic cylinder, can thus be continued.

According to the invention as described in claim 6, pressure oil can besupplied at a flow rate, which corresponds to the level of the bottompressure of the second hydraulic cylinder, through a communication linefor the acceleration of the second hydraulic cylinder. This makes itpossible to prevent occurrence of a shock which would otherwise takeplace as a result of a sudden change in the speed of the secondhydraulic cylinder during an acceleration.

According to the invention as described in claim 7 or 8, an accelerationof the second hydraulic cylinder can be achieved corresponding to thequantity of an operation of the second control device which operates thesecond hydraulic cylinder. This makes it possible to smoothly acceleratethe second hydraulic pressure.

According to the invention as described in claim 9, 10 or 11, anacceleration of the second hydraulic cylinder can also be achievedcorresponding to the quantity of an operation of the first controldevice which operates the first hydraulic cylinder. This also makes itpossible to smoothly accelerate the second hydraulic pressure.

According to the invention as described in claim 12, an acceleration ofthe second hydraulic cylinder under electrical control can be achieved.

According to the invention as described in claim 13, an acceleration ofthe second hydraulic cylinder can be achieved corresponding to thequantity of an operation of the second control device in theelectrically-controlled, hydraulic driving unit. This also makes itpossible to smoothly accelerate the second hydraulic pressure.

According to the invention as described in claim 14, an acceleration ofthe second hydraulic cylinder can also be achieved corresponding to thequantity of an operation of the first control device in theelectrically-controlled, hydraulic driving unit. This also makes itpossible to smoothly accelerate the second hydraulic pressure.

During a combined operation of boom raising and arm crowding performedby supplying pressure oil to respective bottom chambers of a boomcylinder and arm cylinder, the pressure oil in the rod chamber of theboom cylinder has heretofore been drained to a reservoir when the bottompressure of the arm cylinder becomes high. According to the invention asdescribed in claim 16, the pressure oil in the rod chamber of the boomcylinder can be effectively used for the acceleration of the armcylinder in its extending direction, in other words, for theacceleration of arm crowding, and therefore, digging or like work ofearth by this combined operation of boom raising and arm crowding can beperformed with good efficiency.

1. A hydraulic driving unit mounted on a construction machine and provided with a main hydraulic pump, a first hydraulic cylinder and second hydraulic cylinder driven by pressure oil delivered from said main hydraulic pump, a first directional control valve for controlling a flow of pressure oil to be supplied from said main hydraulic pump to said first hydraulic cylinder, a second directional control valve for controlling a flow of pressure oil to be supplied from said main hydraulic pump to said second hydraulic pump, a first control device for selectively controlling said first directional control valve, and a second control device for selectively controlling said second directional control valve, wherein: said hydraulic driving unit is provided with a communication controller to bring a rod chamber of said first hydraulic cylinder and a bottom chamber of said second hydraulic cylinder into communication with each other when a bottom pressure of said second hydraulic cylinder has increased to a high pressure equal to or higher than a predetermined pressure.
 2. A hydraulic driving unit according to claim 1, wherein said communication controller comprises: a communication line capable of bringing said rod chamber of said first hydraulic cylinder and said bottom chamber of said second hydraulic cylinder into communication with each other, a check valve arranged on said communication line to prevent a flow of pressure oil from said bottom chamber of said second hydraulic cylinder toward said rod chamber of said first hydraulic cylinder, and a switching valve for communicating said communication line to a reservoir when said bottom pressure of said second hydraulic cylinder is lower than said predetermined pressure and for maintaining said communication line in a communicating state when said bottom pressure of said second hydraulic pressure has become equal to or higher than said predetermined pressure.
 3. A hydraulic driving unit according to claim 2, wherein said hydraulic driving unit is provided with a detector to detect said bottom pressure of said second hydraulic cylinder, and said switching valve is operated in accordance with said bottom pressure of said second hydraulic cylinder as detected by said detector.
 4. A hydraulic driving unit according to claim 2, wherein said hydraulic driving unit is provided with: a line connected at an end thereof to an upstream side of said switching valve and communicated at an opposite end thereof to said reservoir, and an on/off valve arranged on said line to open said line responsive to a predetermined operation of said first control device.
 5. A hydraulic driving unit according to claim 4, wherein said first control device is a pilot control device for generating a pilot pressure, and said on/off valve is a pilot-controlled check valve.
 6. A hydraulic driving unit according to claim 2, wherein said switching valve comprises a variable restrictor.
 7. A hydraulic driving unit according to claim 2, wherein said hydraulic driving unit is provided with a first flow rate control to control a flow rate through said communication line in accordance with a quantity of an operation of said second control device.
 8. A hydraulic driving unit according to claim 7, wherein said first flow rate control comprises a variable restrictor.
 9. A hydraulic driving unit according to claim 7, wherein said hydraulic driving unit is provided with a second flow rate control to control a flow rate through said communication line in accordance with a quantity of an operation of said first control device.
 10. A hydraulic driving unit according to claim 9, wherein said second flow rate control comprises a variable restrictor.
 11. A hydraulic driving unit according to claim 9, wherein said first control device is a pilot control device for generating a pilot pressure, said switching valve is a pilot-controlled switching valve with a variable restrictor incorporated therein, and said second flow rate control comprises a control line for bringing said first control device and a control compartment of said pilot-controlled switching valve into communication with each other.
 12. A hydraulic driving unit according to claim 2, wherein said communication control comprises: a bottom pressure detector for detecting said bottom pressure of said second hydraulic cylinder and outputting an electrical signal, and a controller for outputting a control signal to selectively control said switching valve in accordance with said signal outputted from said bottom pressure detector.
 13. A hydraulic driving unit according to claim 12, wherein said hydraulic driving unit is provided with a first operated-quantity detector for detecting a quantity of an operation of said second control device and outputting an electrical signal, and said controller comprises: a first function generator for outputting a value such that said value gradually becomes greater as said bottom pressure of said second hydraulic cylinder becomes higher, a second function generator for outputting a value such that said value gradually becomes greater but not beyond 1 as an upper limit as said quantity of said operation of said second control device becomes greater, and a first multiplier for performing multiplication to output said control signal in accordance with a signal outputted from said first function generator and a signal outputted from said second function generator.
 14. A hydraulic driving unit according to claim 13, wherein said hydraulic driving unit is provided with a second operated-quantity detector for detecting a quantity of an operation of said first control device to output an electrical signal, and said controller comprises: a third function generator for outputting a value such that said value gradually becomes greater but not beyond 1 as an upper limit as said quantity of said operation of said first control device becomes greater, and a second multiplier for performing multiplication to output said control signal in accordance with a signal outputted from said first multiplier and a signal outputted from said third function generator.
 15. A hydraulic driving unit according to claim 12, wherein said switching valve is a pilot-controlled switching valve, and said hydraulic driving unit is provided with: an electrohydraulic converter for outputting a control pressure corresponding to a value of said control signal outputted from said controller, and a control line through which said electrohydraulic converter and a control compartment of said pilot-controlled switching valve are communicated with each other.
 16. A hydraulic driving unit according to claim 1, wherein said first hydraulic cylinder and said second hydraulic cylinder comprise a boom cylinder and an arm cylinder, respectively, said first directional control valve and said second directional control valve comprise a center-bypass directional control valve for a boom and a center-bypass directional control valve for an arm, respectively, and said first control device and said second control device comprise a boom control device and an arm control device, respectively.
 17. A hydraulic driving unit according to claim 1, wherein said construction machine is a hydraulic excavator. 